Combustion control in aircraft engines



Oct. 3, 1961 D. R. DE BOISBLANC 3,002,339

COMBUSTION CONTROL IN AIRCRAFT ENGINES Filed Dec. 21, 1956 FUEL CONTROL 1|||||J C n R. OM T O R M WM N NO 1 0 0 AT NM MW 10 q 3 C B R 3 NH C 0 W1 am W M H x w 9 3 R a |r M m U w H C I 5 h m M 4 m T a J 4 2 4 k v 3 a G A F 0 3 ATTORNEYS Patented @ct. 3, 1961 ware Filed Dec. 21, 1956, Ser. No. 629,955 1 Claim. (Cl. 60-356) This invention relates to control systems for regulating combustion in aircraft engines.

This application is a continuation-in-part of copending application Serial No. 220,114, filed April 9, 1951, now Patent 2,799,136.

In various types of aircraft engines, considerable difiiculty has been encountered in attempting to maintain combustion at a desired rate in the combustion chamber. An example of this occurs in reciprocating piston internal combustion engines. Engines of this type are sometimes provided with augmenters, such as described in US. Patent 2,401,941, for example, through which the exhaust gases from the engine cylinders are directed. These exhaust gases provide additional thrust and also draw atmospheric 'air through the augmenter for still additional thrust.

In engines of this type, it is important to regulate the fuel-air ratio so that flame is not present in the augmenter. A flame at this region indicates that the engine is not operating efliciently and may result in an explosion because unburned combustion gases are often present in the augmenter. Inaccordance with the present invention, a combustion control system is provided to prevent flame from reaching the engine augmenter. A flame detecting element is positioned in the engine exhaust to provide a signal representative of flame at this region. This signal actuates a servo system to adjust the fuel-air ratio in the engine to eliminate flame in the region of the detector. The fuel-air ratio can conveniently be adjusted by regulating the rate at which fuel is supplied to the engine.

Accordingly, it is an object of this invention to provide a control system for regulating combustion in reciprocating piston internal combustion engines.

Another object is to provide a control system for preventing flame in the exhaust gases from an internal combustion engine.

A further object is to provide a control system to minimize explosion dangers in internal combustion engines provided with augmenters.

Other objects, advantages and features of this invention should become apparent from the following detailed description which is taken in conjunction with the accompanying drawing in which:

FIGURE 1 is a cross-sectional view of a flame detecting element which can be employed to advantage in the control system of this invention.

FIGURE 2 is a schematic representation of the control system of this invention applied to an internal combustion aircraft engine.

FIGURE 3 is a schematic circuit diagram of the servo system of FIGURE 2.

Referring now to the drawing in detail and to FIGURE 1 in particular, there is shown a sensing probe which can be employed effectively to indicate the presence of flame. This probe comprises a metallic casing which has a metallic electrode 11 positioned therein and electrically insulated from casing 10 by means of supports, such as 12. Electrode 11 is embedded within a tip 14 of a ceramic material which is disposed across one end of casing 10 and which makes electrical contact therewith. Ceramic tip 14 preferably is constructed of some type of refractory material, such as aluminum oxide, which can be formed in a paste by mixing powdered aluminum oxide with water. The paste is molded in a form of a tip across the end of casing 10 and then sintered. Casing 10 is provided with a plurality of holes 15 into which the paste is anchored. This ceramic probe can be of the type more fully disclosed in US. Patent 2,768,266.

It has been found that in an electrical resistance element, such as tip 14, there normally exists a substantially constant distribution of potential at all points through the element. If a charged particle, such as ions which are produced by the reaction of a flame, impinges upon the surface of this resistance element, the constant potential conditions is disrupted so that current flows through the element until all points are again at a common potential. During the time the current is flowing, the region of resistance element in the neighborhood of electrode iii exhibits a potential variation with respect to the reference electrode 10 which depends upon the magnitude of the charge to the particle striking the element, the resistance of the element, and the geometric relationship of electrode 11 with respect to electrode 16. This potential variation can be amplified to provide a detectable electrical signal which is representative of flame impinging upon tip 14. A flame detector of this type can be employed to advantage in aircraft engines because the tip is capable of withstanding the high temperatures which normally are encountered in such engines. The probe responds solely to flame, not to high temperatures alone. Casing 10 can be formed of stainless steel or other materials'which are capable of withstanding high temperatures.

In FIGURE 2, there is illustrated a reciprocating piston internal combustion aircraft engine which has an augmenter attached thereto to provide added thrust. A radial air-cooled internal combustion engine of the type conventionally employed for the propulsion of aircraft is represented schematically by numeral 17. This engine drives a propeller assembly 18. Each engine cylinder, or small group of cylinders, is provided with an exhaust conduit, two of which are indicated by numerals 1? and 29. These conduits extend rearwardly of engine 17 and can be grouped as a cluster of individual conduits or can be merged into a common exhaust pipe 21, as illustrated. Engine 17 is shown as being enclosed within a streamline nacelle 22. An augmenter 24 is positioned rearwardly of exhaust pipe 21 to provide additional thrust from the hot exhaust gases. Augmenter 24 is attached to nacelle 22 by means of struts 25. Atmospheric air is drawn into the augmenter through openings and is therein mixed with the hot exhaust gases. The resulting mixture passes rearwardly through the discharge nozzle 27 to provide added thrust. The thrust provided by means of this augmenter is greater than the thrust provided by the hot exhaust gases alone.

Because the exhaust gases from engine 17 often contain unburned fuel if the fuel-air mixture to the engine is too rich, there is a danger of an explosion taking place within augmenter 24. To minimize this danger, a flame sensing probe 30 is disposed within augmenter 24. The probe can be attached directly to the wall of augmenter 24 which provide a reference ground potential. The center electrode of the probe is insulated from the wall of augmenter 24 by means of an insulating seal 29. The potential difference generated across the probe by flame impinging thereon is amplified by an amplifier 31. The output signal of amplifier 31 energizes a servo control unit 32, which in turn regulates a valve 33 in a conduit 34 which supplies fuel to engine 17. The flow of fuel through conduit 34 normally is regulated by a manually operated fuel control throttle 35 which actuates valve 33 through a friction clutch 36. However, if flame should be detected by probe 30, servo unit overrides the manual control to reset valve 33 in such a manner as to eliminate flame from the exhaust gases.

The-presence of flame in augmenter 24 generally in dicates that the fuel-air ratio to the-engine is too rich. This condition is corrected by decreasing the flow through fuel conduit 34. It should be evident that the location of flame detector 30 can vary somewhat with different types of aircraft engines. In some applications, efiectivc control can be accomplished by positioning the flame detector closer to the engine exhaust, or even in the exhaust pipe 21. The control system of this invention permits the engine to be operated so that flame is effectively eliminated from the engine exhaust. This minimizes dangers from explosions and permits the engine to be operated at higher efficiency.

in FIGURE 3, there is shown a schematic circuit drawing of a suitable servo control system 32. The first output terminal of amplifier 31 is connected through a rectifier 40 to the first terminal of a resistor 41. The second output terminal of amplifier 31 is connected to the second end terminal of resistor 41. A filter capacitor 42 is connected in parallel with resistor 41. Th'e'fluctuating out- "ut signal of amplifier 31 is thus rectified'to provide a direct potential across resistor 41 which is representative of flame striking probe 3b. This potential is connected in opposition to a reference potential, and the resulting po tential diflerence is applied to a chopper circuit 43. The reference potential is supplied by a potentiometer 44 which has a voltage source 45 connected thereacross. Even in the absence of flame, therem'ay be some background noise produced which results'in a small potential across resistor 41. Potentiometer 44 is set to provide a reference'potential of this value. Chopper 43 compares the two potentials and provides an output alternating signal of amplitude and phase representative of the diflerence between the two potentials. This signal is amplified by an alternating current amplifier '46 which energizes a balance motor 47. Mortor 47 is mechanically connected to fuel control valve 33 througharnagnetic clutch 37. The balance motor thus adjusts valve 33 to reduce the fuel supplied to the engine to the extent required to eliminate flame in the region of probe 30. This particular type of servo system is well known in the art and can be of the form described in detail in The Electronic Control Handbook, Batcher and Moulic, Caldwell-Clements, Inc., New York (1946), page 298. 7 Clutch 37 is actuated by a current source 38 which is connected thereto through a relay 39. Relay 39 is connected through a diode 48 to 4 the output of amplifier 31. Thus, motor 47 adjusts valve 33 only when the output of amplifier 31 is large enough to actuate relay 39. This occurs when flame is detected by probe 3%. in the absence of flame, normal control is exercised by throttle 35.

While the invention has been described in conjunction with a present preferred embodiment, it should be evident that it is not limited thereto.

What is claimed is:

In an internal combustion aircraft engine wherein fuel and air are introduced into the engine so that the fuel is burned to provide a source of power and exhaust gases from the engine are directed through an augmenter to give thrust to the engine; a control system comprising means to regulate the introduction of fuel into the engine comprising a valve in a fuel line, means to adjust said valve, and a friction clutch between said means to adjust and said valve; a flame sensing element positioned within the augmenter, said flame sensing element comprising a probe having an electrically conductive casing, an electrode positioned within and electrically insulated from said casing, a tip of ceramic material positioned across one end of said casing and making electrical contact With said casing and with said electrode, and means to provide a signal representative of voltage fluctuations between said casing andsaid electrode responsive to flame impinging on said tip; and means'responsive to said flame sensing means to override said means to regulate so-as' to prevent flame from impinging on said element, said means to override comprising a motor, means including a second clutch to connect said motor to said valve, means responsive to the output signal of said flame sensing element exceeding a preselected value to actuate said second clutch to connect said motor to said valve, and means responsive to said flame sensing element to drive said motor to tend to close said valve.

References (Iited in the file of this patent UBHTED STATES PATENTS Re. 20,471 Kelty Aug. 17, 1937 2,401,941 Lee June 11, 1946 2,538,642 Gardiner et al. Ian. 16, 1951 2,742,756 De Boisblanc Apr. 24, 1956 2,774,215 Mock et al. Dec. 18, 1956 2,795,777 Marsden June 11, 1957 2,799,136 De Boisblanc July 16, 1957 

